1. Field of the Invention
The present invention relates to an oven for baking batter goods, particularly a wafer oven, having several baking plates assigned to one another in pairs, and at least one heat source for tempering the baking plates.
2. The Prior Art
In a wafer baking oven of this type, for example one known from DE 199 42 806 A1, the baking plates are connected with one another by way of an endless baking tong chain that circulates. In this connection, the rectangular baking plates are connected with one another in pairs, by way of a hinge. The hinge is arranged at one of the short sides, so that the plates can be pivoted between a V-shaped open position and a closed position in which they lie essentially parallel to one another.
In these known wafer baking ovens, baking of the wafers takes place continuously, whereby a liquid batter is poured on between an open pair of baking plates, for example, after which the baking plates are closed and locked. The water contained in the batter evaporates between the heated baking plates, and thereby a wafer is formed, under pressure, between the baking plates. The wafer is baked to completion between the baking plates by supplying additional heat. At the end of the circuit described by the pairs of baking plates, the plates are opened again, so that the finished wafer sheet can fall out and be passed to further processing.
In this connection, heating of the baking plates generally takes place by way of direct gas firing. For this purpose, a burner group is arranged under each conveyor segment of the gripper chain. In this way, a first baking plate, the bottom plate, is heated in the upper segment, and the baking plate assigned to or associated with it, the top plate, is heated in the upper segment.
The burner groups consist of pipes that supply a gas mixture to the burners, which can pivot, for example, and are equipped with nozzles. The known wafer oven burners frequently have to be operated with an over-rich gas mixture, i.e. an air/fuel mixture that does not contain sufficient oxygen for complete combustion, since only in this way is it possible to produce a soft, uniform carpet of flames. Consequently, in order to achieve complete combustion, additional oxygen has to be drawn from the air in the oven. This oxygen is sufficiently available for the burner assigned to the lower segment. The burner located in the upper segment, however, is influenced by the waste gases of the bottom burner. In order to produce an acceptable flame profile in the burner assigned to the upper segment as well, fresh air is drawn through the oven using a venting fan. This air is heated on its path through the heated space and, in this connection, transports energy out of the oven, which is therefore not available to the baking process.
The connection between the baking plates by way of the hinge at their short sides frequently makes it necessary for additional reinforcing elements, e.g. casting tongs, to be assigned to the baking plates. But if the distance between the burners and the baking plates is chosen so that the back of the baking plates passes precisely through the hottest part of the flames, i.e. the flame length in the continuous operating state, these reinforcement elements disrupt the flames. This disruption leads to an increase in emissions. In known ovens, the distance of passage through the flames is therefore selected so that the back of the baking plates passes not through the hottest region of the flames, but rather at a greater distance from it.
The effects as described result in a comparatively low degree of effectiveness of about 50% for the known wafer ovens. According to thermodynamic studies, the waste gas losses, which include the air that is additionally drawn through and heated, are approximately 36%. Furthermore, with conventional burners, it is possible to adhere to the waste gas values that apply worldwide, particularly for CO, only under optimal conditions and with a precise adjustment. In most cases, however, the wafer baking ovens produce a multiple of the permissible emissions.
Another disadvantage of the known wafer baking ovens is that only gaseous fuels can be used, but these fuels are not available in sufficient amount at every production location.
Furthermore, it is considered to be disadvantageous that the size of the baking plates is limited to approximately 350 mm×700 mm, since otherwise it is difficult to apply the holding forces for closing the pair of baking plates during the baking process. Making the baking plates larger could, however, clearly increase the production yield while increasing the total machine volume only slightly, and this increase in size would have an advantageous effect on the production costs of the oven, in comparison with its capacity. At the same time, making the baking plates larger also results in energy advantages, since the mass to be heated up, as well as the housing surface, increase relatively less in proportion, in comparison with the capacity increase gained.
Furthermore, wafer baking ovens are known in which the pairs of baking plates are arranged on a rotating shaft or a round table, either parallel or perpendicular to the direction of circulation. Heating of these pairs of baking plates takes place by way of an electrical resistance heater element that is screwed onto them, or by way of hot water or steam that is passed into the baking plates under a pressure of approximately 16 bar, through elongated hole bores. In these known ovens, the number of pairs of baking plates is limited to a maximum of twelve, because of the arrangement in the direct vicinity of the shaft, at a baking plate size of approximately 290 mm×470 mm, so that the capacity of these known systems is low. In addition, when using hot water or steam as the tempering or heating medium, corrosion frequently occurs at the baking plates and the entire system that conducts the medium. This corrosion results in problems, particularly in connection with the high pressure of the tempering medium, approximately 16 bar. Furthermore, the formation of channels within the baking plates is difficult and expensive. The alternative heating of the individual pairs of baking plates by way of electrical resistance heater elements is usually not economically efficient, because of the high costs for electricity.